The present invention relates to an exhaust purification device for an engine, which selectively reduces and purifies nitrogen oxide (NOx) in exhaust by a reducing agent.
In an exhaust system for an engine, an exhaust purification device is known, which includes: a selective reduction catalytic converter that selectively reduces and purifies NOx by using ammonia as a reducing agent; and an injection nozzle that injects a urea aqueous solution (a precursor thereof), the injection nozzle being provided upstream the selective reduction catalytic converter in a flow of the exhaust.
Since the urea aqueous solution is frozen at approximately −11° C., there is known such a technology as described in Patent Document 1, in which, after an engine is stopped, a freezing change of the urea aqueous solution, which remains in the injection nozzle and a reducing agent supply flow path, is monitored, and in addition, the injection nozzle is temporarily opened following this freezing change, whereby such a remaining urea aqueous solution is discharged to an exhaust pipe.
Incidentally, particularly in extremely cold regions, not only the urea aqueous solution in the reducing agent supply flow path but also the urea aqueous solution in a reducing agent tank may be completely frozen in some cases. As countermeasures against this matter, an engine coolant pipe path for flowing an engine coolant that is a type of antifreeze has been routed along the reducing agent tank, the reducing agent tank has been warmed by the engine coolant warmed after the engine is started, and the urea aqueous solution has been thawed. However, it has still taken a long time to thaw the urea aqueous solution even if the above-described countermeasures are taken, and a further improvement has been required.
In view of the above, it is desirable to make it possible to further shorten such a thawing time by providing a structure in which at least a part of the reducing agent or the precursor thereof to be supplied to the injection nozzle is heated and returned to the reducing agent tank.
An exhaust purification device for an engine according to an aspect of the present invention includes: a selective reduction catalytic converter that selectively reduces and purifies nitrogen oxide in exhaust by using a reducing agent, the selective reduction catalytic converter being provided in an exhaust path of the engine; an injection device that injects and supplies a liquid reducing agent or a precursor thereof to an upstream portion of the selective reduction catalytic converter in an exhaust direction; a tank that stores the liquid reducing agent or the precursor thereof, the liquid reducing agent or the precursor thereof being to be injected and supplied from the injection device; a reducing agent supply flow path that supplies the liquid reducing agent or the precursor thereof from the tank to the injection nozzle; a reducing agent circulation flow path that is branched from midway of the reducing agent supply flow path, is capable of returning a part of the liquid reducing agent or the precursor thereof to the tank, and has a part disposed to be capable of exchanging heat with a heat source; a reducing agent circulation valve that controls a flow of the liquid reducing agent or the precursor thereof to the reducing agent circulation flow path; a temperature detection unit that detects a temperature of the liquid reducing agent or the precursor thereof in the tank or the reducing agent supply flow path; and a reducing agent circulation control unit that controls the reducing agent circulation valve so that the liquid reducing agent or the precursor thereof can flow into the reducing agent circulation flow path when the temperature of the liquid reducing agent or the precursor thereof is equal to or less than a predetermined temperature, the temperature being detected by the temperature detection unit.
According to an aspect of the present invention, there is provided the structure in which the at least a part of the reducing agent or the precursor thereof to be supplied to the injection nozzle is heated and returned to the reducing agent tank, and accordingly, it is made possible to further shorten the thawing time.